Upload THIS SKETCH to your M2, which sends a CAN frame on CAN0 or CAN1 when you push switch 1 or 2, respectively. M2 will also blink a light when it receives a CAN frame on either channel.
Connect both M2, A0, and OBD3way board as shown below. This connects the CAN0 of A0 to the CAN0 of M2.
Try hitting the reset button on the A0, upon waking up it will send a CAN frame on CAN0 and the M2 should flash, signifying it has received the frame. Then try sending a frame from the M2 to the A0, the A0 should echo it back to the M2 and it should flash again.